Non-planer, image rotating optical parametric oscillator

ABSTRACT

An Optical Parametric Oscillator (OPO) that includes optical elements located and oriented to form a non-planer, image-rotating ring cavity. To provide a high quality well shaped output beam, the OPO comprises a plurality of reflecting surfaces, designed to rotate the resonating beam by 90 degrees for each round trip in the cavity. Preferred embodiments include a first non-linear crystals and a similar second non-linear crystal mounted side-by-side on a single rotating stage. To minimize the adverse effects of walk-off, a reflecting unit is positioned to cause the output of the first crystal to be reflected into the second crystal. The two crystals are aligned so as to cause walk-off produced in the first of the two crystals to be cancelled by opposite walk-off produced in the second crystal.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional PatentApplication, Ser. No. 61/997,742, filed Jun. 7, 2014, and is aContinuation-In-Part Application of Ser. No. 14/121,438 filed Sep. 6,2014, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the general art of non-linear opticalfrequency conversion systems, and in particular to such systems designedfor improved beam quality, high conversion efficiency and high power.

BACKGROUND OF THE INVENTION Optical Parametric Oscillators

An Optical Parametric Oscillator (OPO) is a device employing one or morenon-linear crystals which when pumped by a laser beam defining a pumpwavelength, can generate coherent light at two different and longerwavelengths. The operation of an OPO typically requires a very highlight intensity in the pump beam which is generally supplied by a veryshort pulse laser. In the OPO at least one non-linear crystal (such asBaB₂O₄, LiB₃O₅, LiNbO₃, KTiOPO₄ and others) is placed in an opticalresonator. When the pump laser beam is directed to propagate through thecrystal, a pair of beams (referred to as the signal beam and the idlerbeam) is produced. Energy of the photons in the beams is conserved so:

$\frac{1}{\lambda_{p}} = {\frac{1}{\lambda_{s}} + \frac{1}{\lambda_{i}}}$

where λ_(p) refers to the wavelength of the pump beam, λ_(s) refers tothe wavelength of the signal beam and λ_(i) refers to the wavelength ofthe idler beam. Typically the shorter wavelength beam is referred to asthe signal beam and the longer wavelength beam is referred to as theidler beam.

The generation of the parametric beams (the idler and the signal) in asingle path through the crystal(s) is inefficient and only a smallfraction of the pump beam is converted. In order to construct anefficient and useful device the crystal(s) are typically placed inside aresonator that is designed to oscillate one or both of the parametricbeams inside the cavity, such that it (or they) are amplified insuccessive passes through the crystal(s). The oscillator components ofthe OPO are typically comprised of optical elements designed to providethe required feedback for efficient conversion. The principles of OPOare well known and described in many publications on lasers andnon-linear optics (for example, A. Yariv, Quantum Electronics, 3^(rd)edition, p. 411. John Wiley & Sons, New York). In many of these OPO'sthe wavelengths of the signal beam and therefore the idler beam can betuned over a wide spectral range by varying the orientation of thecrystal with respect to the laser beam, by changing the crystal'stemperature, or by applying a variable voltage across the crystal.Various tuning ranges can be achieved by properly selecting the laser,the non-linear crystal, and the optical components.

Optical parametric oscillators (OPO's) have been recognized as criticaldevices for a wide range of applications. In the early stages they wereused primarily for research applications and as the designs of thesedevices have improved they have been incorporated in instruments thatare used in commercial applications, (e.g., medical Imaging andhyper-spectral Imaging). The transition from research to commercialapplications present more stringent design constrains, and criticaldemands such as high reliability, high damage threshold, robust andcompact designs. Although there have been significant advances in thedesign of OPO devices since they were first invented, a device that willmeet all of these criteria is still needed.

Efficiency & Damage Threshold

The optical parametric process requires that the signal and idler beamspropagate approximately along the same direction as the pump beam. Insimple OPO designs the pump beam traverses the crystal only in onedirection whereas the parametric beam oscillates back and forth. In thiscase the interaction between the pump and the parametric beams islimited to only half the time the parametric photons are oscillatinginside the cavity, resulting in low conversion efficiency. To maximizeefficiency, OPO's have been designed such that the pump beam as well asthe parametric beams transverse the crystal in the same direction. Forexample, the pump beam can be reflected back through the crystal alongthe same path but in the opposite direction to create what is known as adouble path oscillator. (See, for example, Brosnan, Optical ParametricOscillator Threshold and Linewidth Studies. IEEE JQE Vol. 15, No 6, June1979; Guyer, U.S. Pat. No. 5,079,445; Nabor, U.S. Pat. No. 5,781,571,and Zhang, U.S. Pat. No. 6,295,160). However, in a double path the pumpintensity inside the crystals (and some of the optics) is very high andmay result in damage.

This issue can be resolved in a ring oscillator design in which the pumpbeam and the OPO beams propagate in the same direction, in a closed loopring (Margalith, U.S. Pat. No. 5,276,548, incorporated herein byreference). This design results in very high conversion efficiency whileoperating at about twice the damage threshold of linear double pathcavities (i.e. permitting pumping at twice the intensity without opticaldamage). This design is also very robust and relatively insensitive tominor misalignments of the cavity.

Beam Quality

Early OPO designs demonstrated poor beam quality, which was expressed byhigh divergence, typically on the order of 10 mRad, and non-symmetricalbeam profiles. The main reason for the poor beam quality was the highFresnel numbers of these oscillators. Fresnel number is expressed by

$F = \frac{D^{2}}{\lambda \; L}$

where D is the pump beam diameter, A the resonated wavelength, and L theround-trip length of the cavity. The beam diameter is relatively largeto minimize risk to damage whereas the cavity length is kept to aminimum to maximize the number of round trips in the cavity. Insubstantially all conventional OPO cavity designs, increasing the ratioof beam diameter to cavity length reduces the beam quality. There aretwo methods to improve the beam quality of a short oscillator withrelatively large beam diameters:

-   -   One method is to use a confocal unstable resonator in which        light originally oscillating near the cavity axis gradually        spreads over the entire beam diameter by diffraction and        magnification. This design is limited to a single pass through        the crystals and therefore the efficiency of such an OPO is very        low.    -   Another method is to design an image rotation resonator in which        the OPO beam rotates by 90 deg in each round trip in the cavity.        This technique is covered in details by Arlee V. Smith and        Mark S. Bowers in “Image-rotating cavity designs for improved        beam quality in nanosecond optical parametric oscillators”, J.        Opt. Soc. Am. B/Vol. 18, No. 5/May 2001, P. 706. The paper        provides in-depth analysis of Image rotating oscillators and        offers a number of designs of such oscillators. These designs        have all been studied through simulations. Most of the proposed        oscillators have not been reduced to practice and, to        Applicant's knowledge, the ones that have been reduced to        practice have utilized a linear double path design, which        increases the risk for damage. A true image-rotating ring        oscillator is presented in U.S. Pat. No. 6,775,054 (Smith et al        2004, incorporated herein by reference). This prior art patent        presents an oscillator which incorporates 4 mirrors and a        waveplate that rotates the image of the signal beam in each        round trip while maintaining the polarizations as required. The        4 mirrors were all arranged at a precise 32.8 degrees angle of        incidence. The objective of this invention was to obtain high        efficiency as well as good beam quality. However, in order to        achieve these objectives, the cavity design imposes constrains        which limit its practicality. The device operates at a high        efficiency, but it is most suitable as a single wavelength OPO        as it is difficult to design one that will generate a wide range        of wavelengths, as desired by most applications. The inventors        in that patent show a design in which the two crystals are        placed in series in the same path to increase the gain and        minimize the walk off. With this arrangement tuning would be        very difficult requiring two separate tuning mechanisms. Tuning        issues are not dealt with in the patent. For these reasons very        few devices of this design have been introduced to the market.

Walk-Off Issues

As explained in U.S. Pat. No. 5,276,548 granted to Applicant, incritical phase matching configurations walk-off (which results as beamsof different polarizations propagate through a birefringent crystals)can severely limit the effective gain in OPO devices. This patent ishereby incorporated herein by reference. This problem can be eliminatedby arrangements that pass the beams twice through the same crystal sothe walk-off on the second pass cancels the walk-off on the first pass.The same result can be achieved by causing the beams to pass through twoidentical crystals properly aligned so that the walk-off in one crystalcancels the walk-off in the other crystal.

Other Issues

The performance of an OPO is characterized by various parameters such aswavelength tuning range, conversion efficiency, high damage threshold,spectral line width, and beam quality. Other attributes such as lowcost, long-term stability, robust design and ease of operation areimportant in making the OPO a practical device. For a given pump beam,the design of the oscillator dictates the performance of the OPO.Various-constraints affect the design, and therefore the performance ofpresently available OPO devices.

What is needed is an OPO that not only meets the objectives discussedabove, but also enables wide wavelength tunability, a simple low-cost,reliable, easily operated means to orient two crystals in the cavitywhile maintaining phase matching, high conversion efficiency, highdamage threshold and good beam quality.

SUMMARY OF THE INVENTION

The present invention provides optical parametric oscillator forconverting, with one or more nonlinear crystal, a laser pump beam into asignal beam and an idler beams. The pump beam is injected into anon-planer ring resonance cavity using a mirror that is designed toreflect the pump beam while transmitting the OPO beams. The cavityincludes a nonlinear crystal unit pivotally positioned within the cavityso as to receive the pump beam and to convert energy of the pump beaminto the signal and idler beams with both signal and idler beamspropagating through the crystal unit in directions common to the pumpbeam. The cavity also includes a number of reflecting elements togetherproviding at least six reflecting surfaces. At least four of thereflecting elements are oriented to rotate one or both OPO beams by 90degrees on each pass of the OPO beam or beams through the cavity. Atleast two of the reflecting surfaces are oriented to receive the pumpbeam and the OPO beams after each first pass through a first portion ofthe nonlinear crystal unit and to reflect the pump beam and the OPObeams so as to cause the beams to pass a second time through a secondportion of the nonlinear crystal unit in order to cancel, on the secondpass, walk-off produced by the first pass. The rotation of the OPO beamsalso rotates the polarization of the rotated beams. Therefore inpreferred embodiments the cavity also includes a polarization correctingelement positioned within the cavity and adapted to rotate thepolarization of the one or both rotated beams back to its or theirun-rotated polarization on each pass, through the cavity. In thesepreferred embodiments the wavelengths of the signal and idler beams aredependent of the pivot position of the nonlinear crystal unit and thewavelengths of the signal and/or the idler beams are controlled by thepivoting of the nonlinear crystal unit. In a preferred embodiment thecavity consists of no more than 6 reflecting surfaces (and a beamsplitter to introduce the pump beam into the cavity.

The nonlinear crystal unit may be a single crystal or two crystalsmounted together as a unit and rotated in order to generate differentwavelengths in the OPO. The two reflecting surfaces, oriented to receivethe pump beam and the OPO beams after each first pass through a firstportion of the nonlinear crystal unit and to reflect the pump beam andthe OPO beams so as to pass through a second portion of the nonlinearcrystal unit in order to cancel, on the second pass, walk-off producedby the first pass may be two surfaces of a prism, such as a roof prismor two mirrors. In preferred embodiments, the crystal unit is adaptedfor type II OPO operation. The oscillator may be operated to oscillatethe signal beam or it may be operated to oscillate the idler beam orboth. In a prototype unit built and operated by the Applicant thepolarization correcting element is an achromatic half waveplate and thepump beam is a third harmonic 355 nanometer beam produced by a 1064nanometer Q-switched Nd-YAG laser. The pump beam could also be thesecond harmonic 532 nanometer beam produced by the Q-switched Nd-YAGlaser, or the fundamental wavelength of the Nd:YAG laser at 1064 nm. Thepump laser is not limited to Nd:YAG laser and any laser that meets thecriteria required to pump an OPO, and its harmonics, can serve as thepump. To increase the efficiency of the oscillator, one or morenonlinear crystals may be placed outside the OPO cavity in the outputbeam to amplify the OPO energy.

This invention provides an OPO with unique attributes: wide wavelengthtunability, a simple low-cost, reliable, easily operated means to orientthe nonlinear crystal unit in the cavity while maintaining phasematching, high conversion efficiency while maintaining high damagethreshold and good beam quality. This design is easy to fabricate andtherefore makes OPO's of the present invention practical for a widerange of applications. In preferred embodiments, including Applicant'sprototype, the OPO utilizes a non-planer ring-cavity design whichrotates the signal beam image 90 degrees on each round trip. Thepolarization of the signal beam however is controlled so that, ahead ofeach pass through the non-linear crystals, the polarization of the beamis rotated back to its original orientation. In the prototype embodimentthe ring cavity includes six reflective surfaces including four mirrors,a roof prism (also known as a right angle prism). In addition a dichoticmirror is placed inside the ring cavity to introduce the pump beam intothe cavity. A half wave plate corrects the polarization of theoscillating OPO beam. The prototype device includes a first non-linearcrystal and a similar second non-linear crystal mounted side-by-side ona single rotating stage. The roof prism is positioned to cause the beamexiting the first crystal to be reflected into the second crystal andthe two crystals are aligned so as to cause walk-off in the first of thetwo crystals to be cancelled by opposite walk-off in the second crystal.This OPO is designed to provide a high-energy output beam with excellentbeam quality without damage to the OPO optics. In one form of theinvention, the optical elements include a right angle prism to reflectthe beam exiting the first crystal into the second crystal, twonon-linear crystals, an output coupling mirror and 3 mirrors to reflectthe signal beam in a closed ring. The six reflecting surfaces are twoback surfaces of the prism, the output coupler, and the three mirrorsdesigned to rotate the resonating signal beam. In other preferredembodiments the oscillator could be designed to resonate the idler beamin which case the idler beam would be rotated. Rotation of theresonating beam greatly improves the quality of the output beam.

Applicant has constructed a prototype OPO that was pumped at 355 nm andgenerated very good beam quality, and with this prototype he has proventhat oscillators designed in accordance with the present invention canoperate over the entire visible spectrum and into the infrared region(410 nm to 2400 nm). The OPO can be designed to generate otherwavelengths based on the choice of crystals and the pump laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a preferred embodiment of the present invention.

FIG. 2 high lights and distinguishes an OPO plane and a beam rotationplane.

FIG. 3 shows how five mirrors and a roof prism are used in the preferredembodiment to rotate the image of the signal beam 90 degrees on eachpass through a pair of non-linear crystal.

FIG. 4 shows a design similar to the design shown in FIG. 3 with some ofthe mirrors replaced by prisms.

FIGS. 5 and 6 show additions and variations to the embodiment shown inFIGS. 1 through 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The following is a description of an OPO having a resonance cavity inwhich only the signal beam is resonating: (The same resonator can bedesigned to oscillate the idler beam by selecting a different outputcoupler and exchanging the term signal with the term or with somemodifications both the signal and idler beams could be oscillated.) Theoscillator presented in FIG. 1 is formed by six reflective surfaces. Aroof prism 10, which provides two reflective surfaces, three mirrors 11,12 and 13 that are designed to reflect the signal beam, and mirror 15with a dielectric coating designed to partially transmit the signal beamwhile maximizing the transmission of the idler beam and the pump beam.Together the four mirrors 15, 11, 12 and 13 are arranged to rotate thecross sectional image of the rotated beam or beams by 90 degrees on eachpass through the crystal unit. The pump beam 18 is introduced into thecavity by a mirror 21 that is designed to reflect the pump beam andtransmit the signal and the idler beams. This mirror is not a part ofthe oscillator and serves only to inject the pump beam into the cavity.The cavity incorporates two non-linear crystals 16 and 17 that aremounted side by side on a rotation stage such that the walk-off incrystal 16 is cancelled by walk-off in crystal 17 in each round trip.The two crystals are rotated together around a common axis 20. In thepreferred embodiment the crystals are cut for a type II OPO operation.

The pump beam 18 introduced into the cavity by reflection off mirror 21transverses through crystal 16 and is turned around by 180 degrees bytwo total internal reflections inside the roof prism 10, then the pumpbeam, along with signal and idler components, pass through crystal 17.The two crystals are substantially identical, phase matched andpositioned such that walk-off of the beams generated in the secondcrystal cancels the walk-off generated in the first crystal. The pumpbeam is provided by a short pulse laser which pulses may be about 5 ns.The resonance cavity in preferred embodiments may have a length of about20 centimeters so the OPO beam or beams may undergo 7 or 8 round tripsduring each pulse of the pump beam. The residual pump beam is almostentirely rejected from the cavity by passing through mirror 15, sonearly all of the pump beam passes only once through the crystal unit.This is an important factor controlling the pump energy below the damagethresholds of the crystals and other optical elements in the cavity. TheOPO beams are generated in the crystal unit, propagate in the samedirection and are collinear with the pump beam (in the directions markedby the arrows) between mirrors 21 and mirror 15.

The signal and idler beams and the pump beam are reflected by prism 10(total internal reflection, independent of wavelength). A fraction ofthe oscillating signal beam is reflected back into the cavity, towardmirror 11 by the partial reflector 15 (that serves as the outputcoupler) whereas the majority of the signal beam as well assubstantially all of the entire idler beam and the pump beam aretransmitted through it. The residual pump beam may be separated from theOPO beams by a beam splitter placed outside the OPO (not shown). Most ofthe signal beam, almost all of the idler beam are the output of the OPOas shown at 22. The idler beam may be separated from the signal beamwith a polarizer or with one or more dichroic mirrors. The portion ofthe signal beam that has been reflected by mirror 15 is deflectedupwards 90 degrees in a direction approximately perpendicular to theplane formed by the pump beam (see 25 in FIG. 2). It is then reflected90 degrees by mirror 11 in a direction parallel to the OPO plane 25 andat an angle of 45° relative to the direction of the output beam as shownat 22 in FIG. 1, then reflected downwards by 90 degrees by mirror 12towards mirror 13 where it is reflected again by 90 degrees with mirrors12 and 13 positioned to assure that the rotated beam reflected frommirror 13 is directed in a direction aligned with bump beam 23.

The pump beam preferably is linearly polarized and enters the crystalunit as an extraordinary beam. The optical axes of the crystal unit isin the same plane as the polarization vector of the pump beam and isorientated at an angle to the direction of the pump beam which angle ischosen or adjusted to control the wavelength of the OPO beam or beams.Being an extraordinary beam, the pump beam exhibits a walkoff anglewhich is determined by the pump beam wavelength, the birefringence ofthe crystal and the relative angle between the optical axis and thepolarization vector of the pump beam. In this configuration, the walkofftakes place in a plane perpendicular to the OPO place; however thewalkoff as explained above is reversed as the pump beam is reflectedback by roof prism 10 as shown in the figures' and propagates backthrough the crystal unit in the opposite direction. Wavelengthtunability of the signal beam (and, if desired, the idler beam) isachieved by pivoting the crystal unit about axis 20 which isperpendicular to the optical axis and parallel to or preferably alignedwith the OPO cavity plane.

Additional clarification of the beams path is presented in FIG. 2. Allthe beams propagate along two planes: the OPO plane 25, and the beamrotation plane 35. The pump beam 18 enters the cavity and stays on theOPO plane (25) until it leaves the cavity through the output couplingmirror (15) along the path 22. The portion of the signal beam that isreflected by the output coupling mirror 15 propagates in the imagerotation plane 35 as shown in FIG. 2. The two planes are perpendicularto each other, and the Image rotation plane forms a 45 degrees angle 27with the direction of the both the transmitted beam direction 29 and thepump beam direction 30, both which beams are propagating in the OPOplane 25.

A critical attribute of this non-planner OPO design is that it forcesthe beam to rotate by 90 degrees each round trip in the cavity. Thisrotation results with high quality, low divergence OPO beams. The beamrotation can be visualized by following the change of orientation of thesymbol F, which represents an arbitrary cross sectional image of thebeam plane, as the beam is reflected by the oscillator optics. Startingin position 1 and propagating clockwise, the beam is rotated by theprism (10) to a new orientation in position 2. The beam image, presentedby the symbol F, keeps changing orientation after each reflection. Ascan be seen in the figure, after it completes the trip in the cavity andreaches position #5, it is at 90 degrees with respect to the image of Fin the starting position #1.

The polarization of the signal beam is also rotated 90 degrees togetherwith its image, but it is rotated back by 90 degrees on each pass as thebeam propagates in the cavity. This is accomplished with an achromatichalf waveplate 24 which is shown in the drawings as being positionedbetween mirrors 11 and 12 but it could be positioned anywhere betweenmirror 15 and the non-linear crystal unit.

The pump beam 18 is propagating in a plane 25 as shown in FIG. 1. It islinearly polarized perpendicularly to the plane. The pump beams passesonce through each crystal 16 and 17 and exit the cavity 22. It maintainsthe polarization and its image orientation as it traverses the cavitybetween optics 21 and 15. The two crystals are mounted side-by-side andare rotated together around a common axis 20. They can be rotated forwavelength tuning by a single mechanism while maintaining the phasematching between them. This configuration allows the tuning of thetwo-crystal unit without a need for two motors or a gear. In applicantsprototype embodiment the crystals are rotated about 40 degrees to scanthe signal beam from about 410 nm to about 2400 nm.

This non-planar ring cavity presented in this invention has all theadvantages presented by the ring oscillator of U.S. Pat. No. 5,276,548,with the addition of an image rotation feature. This OPO is not muchlarger than the planer ring design and therefore, the number of roundtrips in the cavity will be similar when pumped by a laser with the samepulse length. This design results in very good beam quality, lowdivergence, high conversion efficiency, and high damage threshold OPO.In addition the OPO provides wide wavelength tunability, a simple,low-cost, reliable, easily operated mechanism to orient the two crystalswhile maintaining phase matching.

Variations

Persons skilled in this art will recognize that many variation of andadditions to the specific design described in detail above are possibleutilizing the novel concepts of the present invention. For example asshown in FIG. 4 mirrors 11, 12 and 13 could be replaced by roof prismsarranged to provide 90 degree reflections which provide almost totalinternal reflection. Also the two substantially identical non-linearcrystals as shown in FIG. 1, could be replaced by a single largercrystal 29 as shown in FIG. 5. Applicant prefers to use the two smallercrystals because two smaller crystals are typically less expensive thanone larger crystal required for the same OPO task. Another variationwould be to add one or more crystals, 36 outside the cavity to serve asan amplifier as shown in FIG. 6. At this position in the signal beamwill be further amplified by extracting additional energy from thecollinear pump beam. Applicant estimates that this addition willincrease the efficiency of the system from about 30 percent to about 50percent. OPO systems of the present invention could also include opticalcomponents to separate the three frequencies in output beam 22.Alternatively this task may be left to the user of the OPO. Additionalvariations that are described in Applicants U.S. Pat. No. 5,276,548(which has been incorporated herein) could also be utilized in OPOsystems of the present invention.

Therefore the scope of the present invention should be determined by theappended claims and not by the examples that have been given.

What is claimed is:
 1. An optical parametric oscillator for converting,with at least one nonlinear crystal, a laser pump beam into a signalbeam and an idler beam, said signal and idler beams defining two OPObeams, said oscillator comprising: A) a non-planer ring resonance cavitycomprising: 1) a nonlinear crystal unit pivotally positioned within thecavity so as to receive the pump beam and to convert energy of the pumpbeam into the signal and idler beams with both signal and idler beamspropagating in directions common to the pump beam; 2) plurality ofreflecting elements together providing at least six reflecting surfaces,wherein: I) at least four of the reflecting elements are oriented torotate one or both OPO beams by 90 degrees on each pass of the rotatedOPO beam or beams through the cavity, and II) two of the reflectingsurfaces are oriented to receive the pump beam and the OPO beams aftereach first pass through a first portion of the nonlinear crystal unitand to reflect the pump beam and the OPO beams so as to pass a secondtime through a second portion of the nonlinear crystal unit in order tocancel, on the second pass, walk-off produced by the first pass; B) apump beam deflector positioned to deflect the pump beam into theresonance cavity; C) a polarization correcting element positioned withinthe cavity and adapted to rotate the polarization of the one or bothrotated beams back to its or their un-rotated polarization on each pass,of the one or both rotated OPO beams, through the cavity; whereinwavelengths of the signal and idler beams are dependent of the pivotposition of the nonlinear crystal unit.
 2. The optical parametricoscillator as in claim 1 wherein said at least six reflecting surfacesconsists of no more than six reflecting surfaces.
 3. The opticalparametric oscillator as in claim 1 wherein the nonlinear crystal unitis comprised of a single non-linear crystal.
 4. The optical parametricoscillator as in claim 1 wherein the nonlinear crystal unit is comprisedof two substantially similar non-linear crystals mounted together andadapted to pivot as a unit about a common axis.
 5. The opticalparametric oscillator as in claim 3 and further comprising a rotationstage adapted to pivot the two crystals about the common axis to adjustthe wavelength of the rotated beam.
 6. The optical parametric oscillatoras in claim 2 and further comprising a rotation stage adapted to pivotthe single non-linear crystal about an axis to adjust the wavelength ofthe rotated beam.
 7. The optical parametric oscillator as in claim 1wherein the two reflecting surfaces, oriented to receive the pump beamand the OPO beams after each first pass through a first portion of thenonlinear crystal unit and to reflect the pump beam and the OPO beams soas to pass a second time through a second portion of the nonlinearcrystal unit in order to cancel, on the second pass, walk-off producedby the first pass, are two surfaces of a prism.
 8. The opticalparametric oscillator as in claim 7 wherein the prism is a roof prism.9. The optical parametric oscillator as in claim 1 wherein the tworeflecting surfaces, oriented to receive the pump beam and the OPO beamsafter each first pass through a first portion of the nonlinear crystalunit and to reflect the pump beam and the OPO beams so as to pass asecond time through a second portion of the nonlinear crystal unit inorder to cancel, on the second pass, walk-off produced by the firstpass, are two mirrors.
 10. The optical parametric oscillator as in claim1 wherein the crystal unit is adapted for type II OPO operation.
 11. Theoptical parametric oscillator as in claim 1 wherein the oscillator isadapted for oscillation of the signal beam.
 12. The optical parametricoscillator as in claim 1 wherein the oscillator is adapted foroscillation of the idler beam.
 13. The optical parametric oscillator asin claim 1 wherein the polarization correcting element is an achromatichalf waveplate.
 14. The optical parametric oscillator as in claim 1wherein the pump beam is a 1064 nanometer laser beam produced by aQ-switched Nd-YAG laser.
 15. The optical parametric oscillator as inclaim 1 wherein the pump beam source is a second harmonic 532 nanometerlaser beam produced by a Q-switched Nd-YAG laser.
 16. The opticalparametric oscillator as in claim 1 wherein the pump beam source is athird harmonic 355 nanometer laser beam produced by a Q-switched Nd-YAGlaser.
 17. The optical parametric oscillator as in claim 11 wherein theat least four of the at least seven reflecting surfaces are surfaces ofmirrors.
 18. The optical parametric oscillator as in claim 11 where atleast one crystal is placed outside the OPO cavity to amplify the OPOenergy.
 19. The optical parametric oscillator as in claim 1 wherein theat least four of the reflecting elements are oriented to rotate one orboth OPO beams by 90 degrees on each pass of the rotated OPO beam orbeams through the cavity includes at least one prism.
 20. The opticalparametric oscillator as in claim 19 wherein the at least one prism isthree prisms.